1. Field of the Invention
The present invention relates to activated carbon for use in electric double layer capacitors, and a method of manufacturing such activated carbon.
2. Description of the Related Art
When the surface of a solid comes into contact with an electrolytic solution, positive or negative ions are selectively adsorbed to the surface of the solid, positively or negatively charging the surface of the solid. At the same time, an increased number of oppositely charged paired ions are developed in the electrolytic solution. As a result, positive and negative charges are arranged across the interface between the surface of the solid and the electrolytic solution, thereby making up an electric double layer. It is known in the art that the electric double layer can be used as a capacitor with the solid serving as an electrode. Such a capacitor is called an electric double layer capacitor.
In the electric double layer capacitor, the electrostatic capacitance produced when an electric field is applied to the solid electrode is proportional to the surface area of the solid electrode. Generally, electric double layer capacitors generally use a substance having a large specific surface area, such as activated carbon, as electrodes, and the electrodes is impregnated with an electrolytic solution. The electrodes are separated by a separator, and current collectors are attached to the respective electrodes that are disposed one on each side of the separator.
Electric double layer capacitors find use as backup power supplies for electric devices because they have a large capacitance in farads and exhibit excellent charging and discharging cycle characteristics. Efforts are being made to use electric double layer capacitors as a substitute for vehicle-mounted batteries.
As shown in FIG. 1 of the accompanying drawings, an electric double layer comprises a pair of polarized electrodes 1 made of activated carbon which are positioned to face each other across a separator 2. The polarized electrodes 1 are impregnated with an electrolytic solution containing tetraalkylammonium salt or the like dissolved in an organic solvent. The polarized electrodes 1 with the separator 2 sandwiched therebetween are housed in a casing 3 of aluminum. The casing 3 is closed by a lid 5 of aluminum. The casing 3 and the lid 5 serve as current collectors held in contact with the polarized electrodes 1, respectively. Specifically, each of the polarized electrodes 1 is made of a shaped mixture of particles of activated carbon and a binder of tetrafluoroethylene or the like. Alternatively, each of the polarized electrodes 1 may be made of fibers of activated carbon.
In the illustrated electric double layer capacitor, charges in the polarized electrodes 1 close to the current collectors can easily be collected by the current collectors. However, charges in the polarized electrodes 1 remote from the current collectors are reduced due to the internal resistance of the polarized electrodes 1 before they are collected by the current collectors. Therefore, the electric double layer capacitor has a relatively small effective electrostatic capacitance.
Attempts have been made to improve the internal structure of the particles of activated carbon for reducing the internal resistance of the polarized electrodes 1. For example, some polarized electrodes are made of particles of activated carbon which are mixed with a conductive filler such as of furnace black. According to Japanese laid-open patent publication No. 61-187322, fibers of activated carbon for polarized electrodes have an inner region made of graphite and an outer layer of activated carbon. The inventors of the present invention have proposed to form polarized electrodes of a mixture of particles of activated carbon and a conductive filler by carbonizing an easily graphitizable resin and activating the carbonized resin with an alkali for thereby producing particles of activated carbon, each having a laminated structure of graphite extending inwardly from the surface of the particle (see Japanese patent application No. 8-132050).
However, the improved internal structures of the particles of activated carbon have not been sufficiently effective in reducing the internal resistance of polarized electrodes.